In a vehicle power steering system, a number of components are employed to assist in steering the wheels. The principal components are a hydraulic actuator to apply steering assist force to the wheels, a power steering pump to supply hydraulic fluid to the actuator, and a steering gear including a control valve to control the flow of fluid from the pump to the actuator in response to rotation of the steering wheel. Steering wheel rotation relatively moves two parts of the valve to admit fluid flow to the actuator. The steering gear employs a resilient element, such as a torsion bar, which resists the relative movement of the two parts of the valve to give feel or steering effort to the steering wheel.
For a given type of vehicle, it is desirable to have a particular steering effort to meet driver expectations. Manufacturing variables, however, lead to differences from one vehicle to another, so that the steering effort intended for a particular model is not always achieved The variables chiefly arise from the resilient elements in the steering gears which may vary from one unit to another, and the power steering pumps which vary in output volume.
It has been proposed to control power steering effort by combining an electromagnetic actuator with the resilient element, so that the net effort is under electrical control, and may be greater or less than the resilient element alone. One such proposal is disclosed in U.S. Pat. No. 4,871,040 to Zuraski et al. issued Oct. 3, 1989, and assigned to the assignee of the present invention. The Zuraski et al. patent employs both a torsion bar and a magnetic spring comprising an electromagnetic device controlled by an electrical current coupled across the torsion bar to either enhance or counter the effect of the torsion bar as a function of the current magnitude and direction. The current is under control of a microcontroller programmed to vary the current in a prescribed manner. The principal purpose of the device cited by Zuraski et al. is to vary the current with vehicle speed to achieve low steering effort at low speeds and higher steering effort at high speeds. It is now proposed in the present invention to use a controllable steering effort system in the solution of the problem of vehicle-to-vehicle variability.